Thermal and electrical properties of Nb 2 AlC, (Ti, Nb) 2 AlC and Ti 2 AlC
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ernary carbides Nb2AlC and Ti2AlC belong to a larger class of solids with the general formula MN+1AXN , where N ⫽ 1 to 3, M is an early transition metal, A is an A-group (mostly III and IV A) element, and X is C or N. These, so-called MAX, phases are hexagonal (P6/mmc) layered compounds, wherein pure layers of the A-group elements are interleaved with MN+1XN layers having the rock-salt structure.[1] Over the past few years, compounds have been shown to posses unusual combinations of properties.[1–16] They are readily machinable but quite stiff, resistant to thermal shock and damage tolerant. Some of them also exhibit some very attractive high-temperature properties. Nowotny and co-workers[17,18] were the first to synthesize Nb2AlC and Ti2AlC. Later, they reported on the existence of a solid solution between the end members.[19] Recently, details on the synthesis and characterization of Ti2AlC, Ti2AIN, and Ti2AlC0.5N0.5 were reported.[3,9] In the latter work,[9] it was shown that, whereas the mechanical properties and thermal-expansion coefficients (TECs) were affected by the formation of the solid solution, the electrical properties were much less altered. In general, the MAX phases are good thermal conductors because they are good electrical conductors.[5,10,14] The phonon contribution to the total conductivity, with a few notable exceptions, Ti2AlC being one of them, is small, despite the fact that these solids are elastically stiff and have high Debye temperatures.[6,16] The phonon contribution is suppressed because of two factors. First, the presence of small concentrations of point defects which, not unlike the binary nearstoichiometric MX phases, are potent phonon scatterers.[20,21] M.W. BARSOUM, Professor, I. SALAMA, and T. EL-RAGHY are with the Department of Materials Engineering, Drexel University, Philadelphia, PA 19104. Contact e-mail: [email protected] J. GOLCZEWSKI, H.J. SEIFERT, and F. ALDINGER are with the Max Planck Institut fur Metallforschung, Stuttgart, Germany. W.D. PORTER and H. WANG are with the Oak Ridge National Laboratory, Oak Ridge, TN 37831. Manuscript submitted November 9, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
This is one reason, for example, Ti4AlN2.9, is a poorer phonon conductor than Ti2AlC.[1] The second reason is more unique to the MAX phases in that the A element, by virtue of its being relatively loosely bound, acts as a rattler.[1] Rattlers, defined as atoms whose vibrational amplitudes are higher than other atoms in the structure, are also potent phonon scatterers.[22,23] This hypothesis explains why, for example, the phonon contribution to the total conductivity in Ti3SiC2 or Nb2SnC is quite small.[1] As discussed subsequently in Section IIIC, Ti2AlC is an exception because at room temperature roughly 50 pct of the heat conductivity is due to phonons.[1] This article is a continuation of our efforts to fabricate and fully characterize all the known MAX phases.[1,17] Here, we report on the heat capacities, thermal expansion coefficients, TCE’s, TECs, and thermal cond
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